Nitride semiconductor devices are being actively developed as high-withstand-voltage high-power semiconductor devices which take advantage of characteristics such as high saturated electron velocities and wide bandgaps. Among nitride-semiconductor devices, many reports on nitride-semiconductor field-effect transistors, especially High Electron Mobility Transistors (HEMTs), have been made. In particular, attention is being focused on AlGaN/GaN HEMTs that have an electron transit layer of GaN and an electron donor layer of AlGaN. In an AlGaN/GaN HEMT, distortion due to the difference in lattice constant between GaN and AlGaN occurs in AlGaN. This causes piezoelectric polarization and spontaneous polarization of AlGaN to provide a high-concentration two-dimensional electron gas (2DEG). Accordingly, high withstand voltage and high power can be achieved.
Non-Patent Document 1: Panasonic Technical Journal Vol. 55, No. 2, 2009
In switching elements used in devices such as power supply devices, the so-called normally-off operation in which no current flows in the absence of voltage is desired in terms of fail-safe. However, in a GaN HEMT that uses high-concentration 2DEG, current flows even in the absence of gate voltage because there are many electrons in the channel of the transistor. To shut off the current, a negative voltage needs to be applied to the gate electrode, and thus the device operates in a normally-on mode.
One proposed approach to implementing a normally-off GaN HEMT is to form a p-type GaN layer or a p-type AlGaN layer on an electron donor layer to control the concentration of 2DEG by a band modulation effect.
The p-type (Al)GaN layer is doped with an acceptor impurity, typically Mg. Since Mg has an activation rate as low as approximately 1%, it is difficult to make a p-type GaN crystal that has a high hole concentration. Therefore, in the case of the p-type (Al)GaN layer formed on the electron donor layer, the p-type (Al)GaN layer needs to be thicken in order to increase the energy level of the conduction band at the AlGaN/GaN hetero interface to a level higher than the Fermi level.
However, thickening the p-type GaN layer increases the distance from the gate electrode to the AlGaN/GaN hetero interface which serves as a channel. Accordingly, response speed decreases and electric fields in the channel from the gate electrode will be insufficient, thereby causing degradations of device characteristics, such as poor pinch-off.